Tmc regulates membrane viscosity in mammalian cochlear hair cells.

Abstract

The hair bundle, composed of rows of actin-filled stereocilia, is the sensory organelle of inner ear hair cells. There is a limited but growing body of data suggesting that stereocilia membrane properties modulate mechanically-gated (MET) ion channels located on the stereocilia. Transmembrane Channel Like proteins (TMCs) are considered part of the MET channel machinery, but also have membrane scramblase activity. To further investigate the role of membrane in modulating MET channel and test whether the presence of TMCs might account for the previously reported differences in stereocilia and soma membrane properties, we used a novel viscosity sensor BODIPY1c. The lifetime changes of BODIPY1c based on viscosity allowed precise spatial and dynamic monitoring of membrane properties within live hair cells. BODIPY1c can also enter hair cells through MET channels and fluorescently label the cytoplasmic membranes, thus allowing to identify hair cells with functional MET channels. Using synthesized liposomes of sizes ranging from 90-2000 nm with lipid composition matching that of stereocilia, we show that membrane curvature established with stereocilia morphology will not account for the observed difference between stereocilia and soma. We also show that the membrane viscosity of stereocilia and soma of mammalian cochlear hair cells vary during development. Membrane viscosity decreases and strongly correlates with the onset of MET. TMIE and TMC1 mutant mice, both of which lack TMC1 in stereocilia, have significantly higher membrane viscosity compared to litter mate controls at P10. In TMC1 mutants, the stereocilia membrane viscosity strongly correlates to the level at which the bundles are transducing. Inhibition of MET channel current in P10 rats for 30 mins with 1 mM curare, however, did not alter the membrane viscosity suggesting that the MET current is not directly driving the decrease in membrane viscosity.